1. Field of the Invention
The present invention relates to lubricating structures for compressors, and more particularly, to improvements in circulation passages for lubricating oil in compressors that employ swash plates.
2. Description of the Related Art
A typical variable displacement compressor that employs a swash plate has a cylinder bore and a piston accommodated therein. A compression chamber is defined in the cylinder bore by the piston. The piston is coupled to the swash plate by means of shoes. The swash plate is arranged in the crank chamber about a drive shaft. A hinge mechanism supports the swash plate in a manner such that it is inclined in accordance with the difference between the pressure in the crank chamber and the pressure acting on the face of the piston. In this type of compressor, the swash plate is moved to a minimum inclination position at which its inclination becomes minimal with respect to a plane perpendicular to the drive shaft (the state in which the compressor displacement is minimal). When the swash plate is located at the minimum inclination position, lubricating oil, which is contained in a refrigerant, is conveyed from the compression chamber to the crank chamber through a clearance defined between the piston and the wall of the cylinder bore to lubricate the swash plate and the shoes. With regard to the swash plate, a considerable amount of load is applied to a portion corresponding with the hinge mechanism in the axial direction of the drive shaft. The load applied to this portion is greater than the load applied to other portions of the swash plate. Accordingly, it is particularly important that the portion receiving the heavy load be sufficiently lubricated to improve the durability of the swash plate.
The swash plate is provided with a shaft hole to insert the drive shaft therethrough. When machining a workpiece to form the swash plate, a reference hole extending parallel to the shaft hole is provided in addition to the shaft hole. The workpiece, which is cast and disk-like, is secured to a jig. The jig is fixed on a table of a numerically controlled (NC) milling machine. The workpiece is machined by a grinding stone that is attached to a spindle of the milling machine. The workpiece must be fixed to the jig so as to prevent it from rotating when undergoing machining. Thus, a center shaft projecting from the jig is inserted through the shaft hole of the workpiece while a positioning pin projecting from the jig is inserted through the reference hole. In this manner, the workpiece is supported at two locations by the jig to prevent rotation of the workpiece. This enables stable machining of the workpiece when forming the swash plate.
As described above, the lubricating oil contained in the refrigerant is conveyed from the compression chamber toward the crank chamber via the clearance defined between the piston and the wall of the cylinder bore. When the lubricating oil leaks into the crank chamber, the oil advances along the surface of the swash plate toward the shoes and then lubricates between the swash plate and the shoes. However, the refrigerant containing the lubricating oil flows into the reference hole. This affects the flow of the lubricating oil in an undesirable manner. Insufficient lubrication of the region receiving the heaviest load results in early wear of the plate. Such insufficient lubrication is especially troublesome in compressors that do not use clutches (clutchless compressors) such as those described in Japanese Unexamined Patent Publication Nos. 3-37378 and 7-286581.
In a typical clutchless compressor, it is important to prevent excessive compressor displacement when cooling is not required and to prevent frost from forming in the associated evaporator. The circulation of refrigerant through the external refrigerant circuit is stopped when cooling is not required or when there is a possibility of the formation of frost. In the compressors of Japanese Unexamined Patent Publication Nos. 3-37378 and 7-286581, the circulation of refrigerant in the external refrigerant circuit is stopped by impeding the flow of refrigerant gas entering the suction chamber of the compressor from the external refrigerant circuit. In these compressors, when the flow of refrigerant gas from the external refrigerant circuit to the suction chamber is impeded, the swash plate is moved to the minimum inclination position. If the flow of refrigerant gas from the external refrigerant circuit to the suction chamber is commenced, the inclination of the swash plate is increased from the minimum inclination. When the swash plate is located at the minimum inclination position, the refrigerant in the external refrigerant circuit does not return to the compressor. In this case, lubrication of the interior of the compressor is carried out by the lubricating oil contained in the refrigerant that circulates within the compressor. The refrigerant passing through the clearance is part of the refrigerant circulating within the compressor. Thus, when the lubricating oil that is contained in the circulating refrigerant becomes insufficient, it is difficult to avoid early wear since the swash plate is constantly rotated during operation of the external drive source that drives the compressor.